I have to say first that it feels like the author is reacting to the hate on Shion by suggesting it was Mona that gave Shion the initial push to leave Kiki's side and become her partner...
That said, I'm... a bit confused on everyone's reaction here. So many people hate Shion and what she did, but then cheer Mona on and call her best character even after being revealed as the cause of it? I like Mona a lot just to be clear, but that bias is a bit weird to me. Shion put some space between her in Kiki (and not in the best of ways), yet the response throughout the chapters have been far more spiteful than deserved.